This investigation is aimed at determining flow regimes in Y-junctions with flow division. Air velocity and velocity pulsations are measured using a hot-wire anemometer in a wide range of Reynolds numbers Re = 400–6000. A junction with diameter d = 6 mm is chosen as the object of study for the inlet and two outlet channels: a symmetrical arrangement of the outlet channels with an angle of 50 º between them. A long tube with diameter d = 6 mm and length l/d = 333 is connected to the junction inlet. Two techniques of the flow regime diagnostics have been developed. For the inlet channel of the junction, the critical Reynolds number is Re = 2000 (puff-type vortex structures appear for the first time). For two outlet channels, a significant increase in velocity pulsations occurs at lower Reynolds numbers Re = 1640–1660. By analogy with diagnostics of the flow movement in a round tube, the following classification of flow regimes in a junction is proposed: laminar flow at Re < 1640–1660, transitional flow at Re = 1660–2800, and turbulent flow at Re > 3000. The obtained data can be used to clarify the range of data for laminar, transitional, and turbulent flow regimes in round Y-junctions with flow division.